The present invention relates to a biaxially oriented polyester film. More particularly, it relates to a biaxially oriented polyester film having a relatively uniform surface with few coarse projections and also excelling in transparency, flatness, running properties and wear resistance.
Polyester films represented by polyethylene terephthalate films have excellent physical and chemical properties and are popularly used for many applications, such as graphic arts, displays, packages, etc., by taking advantage of their excellent transparency. They are also widely used in the fields of base films of magnetic recording media, capacitor materials and such.
In production of the films which are capable of making the best use of their properties such as transparency, running properties and wear resistance, it is especially required for such films to have good running properties for the reason of facilitating passage through the steps in the manufacturing process, treatments in the after-processing steps such as coating or deposition, and handling of the product itself, but such requirement was not attained sufficiently in the prior art. In most cases, attributable to friction and wear caused by high-speed contact between the film and the base material.
It is known that in order to improve the running properties and wear resistance of the film, generally, the film surface is roughened in an appropriate degree. For attaining this, a method is used in which fine particles are allowed to exist in the starting polyester. Although this method has come in practice in some quarters, it has not yet been realized to satisfy these property requirements to a high degree.
For example, in case where the so-called "precipitated particles" produced from the catalyst residue in the production of polyester are used as fine particles, there arise the problems such as unsatisfactory running properties and wear resistance because such particles are liable to break on film stretching, and difficulties in recycling the used material. In case of adding the inorganic compound particles inert to polyester, such as kaolin, silicon oxide, titanium dioxide and calcium phosphate, the running properties can indeed be improved since the particles won't be broken or deformed by film stretching and are capable of forming the relatively sharp projections. But because of their poor affinity for polyesters, voids may be formed around the particles when the film is stretched, causing a sharp deterioration of transparency, and also the particles tend to come off the film surface, causing formation of "white dust".
In another known particle addition method, there are used the silica particles having relatively good affinity for polyesters (Japanese Patent Application Laid-Open (KOKAI) No. 56-42629). Such particles, however, are liable to deform on stretching, making it hard to form the sharp projections on the film surface, so that the produced film may prove unsatisfactory in running properties. Also, since the silica particles are lump-shaped and have a very wide particle size distribution, many coarse projections exist in the produced film surface to deteriorate its flatness and transparency.
In order to overcome this antinomic phenomenon, use of the inorganic particles having a sharp particle size distribution is proposed recently. For instance, Japanese Patent Application Laid-Open (KOKAI) No. 62-207356 discloses monodisperse spherical silicon oxide particles. In use of these particles, however, because of their poor affinity for polyesters, voids may be formed around the particles depending on the stretching conditions, causing deterioration of transparency or fall-off of the particles.
As viewed above, there is yet available no polyester film which can satisfy the requirements for transparency, flatness, running properties and wear resistance to a well satisfactory degree and is also provided with other necessary properties.
As a result of the present inventor's earnest studies to solve the above problem, it has been found that the said problem can be easily solved by providing a film containing a specified amount of specific silica particles.
The present invention has been attained on the basis of the above finding.